This invention relates to ophthalmic and optical glasses having a low specific gravity and a refractive index (nd) higher than 1.88.
In the past, glasses having a low refractive index (nd) of about 1.52 have been widely used for ophthalmic lenses. In a case where positive power or negative power lenses having a high diopter value are made of a glass having such a low refractive index, the thickness of the lenses in the edge or in the center increases with resulting increase in the weight of the lenses and also increase in chromatic aberration which produces iris in the edge of the lenses. These lenses are disadvantageous to the wearer of the spectacles both in use and appearance. For reducing the thickness of lenses having a high diopter value, glasses having a high refractive index (nd) of about 1.9 may be used. As one of such glasses having a high refractive index, Japanese Patent Application Laid-open Publication No. Sho 52-155614 discloses an optical glass of a B.sub.2 O.sub.3 --La.sub.2 O.sub.3 --Gd.sub.2 O.sub.3 --WO.sub.3 --Zro.sub.2 and/or Ta.sub.2 O.sub.5 system. This glass, however, contains a relatively large total amount of Gd.sub.2 O.sub.3, WO.sub.3 and Ta.sub.2 O.sub.5 resulting in a high specific gravity and, therefore, this glass cannot sufficiently reduce the weight of the lenses. Moreover, this glass tends to be colored and has not sufficient resistance to devitrification. Japanese Patent Application Laid-open Publication No. Sho 48-23809 discloses a high refractive optical glass in which TiO.sub.2, ZrO.sub.2 and Nb.sub.2 O.sub.5 are added to a B.sub.2 O.sub.3 --La.sub.2 O.sub.3 --Gd.sub.2 O.sub.3 --Ta.sub.2 O.sub.5 --ZnO system glass. This glass, however, contains a relatively large amount of La.sub.2 O.sub.3 resulting in a high specific gravity and, moreover, is not sufficient in acid-proof property and resistance to devitrification. Japanese Patent Application Laid-open Publication No. Sho 53-4023 discloses a high refractive optical glass in which HfO.sub.2 is introduced into a B.sub.2 O.sub.3 --La.sub.2 O.sub.3 system glass. This glass also has a high specific gravity because it contains a large amount of La.sub.2 O.sub.3 and HfO.sub.2. Besides, since HfO.sub.2 is a very expensive material, this glass is not suited for a large scale production.
Japanese Patent Application Laid-open Publication No. Hei 6-56462, Japanese Patent Application Laid-open Publication No. Hei 6-87628 and International Application Laid-open Publication WO98/32706 also disclose low density and high refractive glasses but these glasses do not have a sufficient stability against devitrification and therefore are not suited for a large scale production.
Japanese Patent Application Laid-open Publication No. Sho 62-100449 discloses a g lass of a B.sub.2 O.sub.3 --La.sub.2 O.sub.3 --ZnO--Li.sub.2 O--Sb.sub.2 O.sub.3 system. This glass, however, has a low refractive index and, besides, since it contains a large amount of Sb.sub.2 O.sub.3, the Sb.sub.2 O.sub.3 ingredient evaporates in melting of the glass and, as a result, it is difficult to obtain a homogeneous glass.
It is, therefore, an object of the present invention to provide ophthalmic and optical glasses which have eliminated the above described disadvantages of the prior art glasses and have a low specific gravity, a high refractive index and sufficient homogeneity a nd resistance to devitrification.